Some folks are adding an "S" for Starbucks, which has a market
cap of $87 billion and is up 44% year-to-date.

"Just five stocks with outsized returns accounted for the strong
performance of the S&P 500; hundreds of others underperformed
the broader market," Goldman Sachs Asset Management analysts said
in the firm's 2016 outlook report.

They included this chart illustrating market breadth, which they
calculated by considering the weight of each of the S&P 500's
stocks and the six-month returns generated. Because the S&P
500 is a market-cap-weighted index, big companies like Google and
Starbucks have much larger impacts on returns than smaller
companies like Electronic Arts, which has a 46% year-to-date
return but a modest $21 billion market cap.

According to David Kostin of Goldman Sachs' Global Investment
Research team, the average breadth in this index is 35. A breadth
of 5 is considered narrow. The index currently sits at less than
2.

Goldman Sachs Asset Management

"Firstly, we need to point out that market breadth is NOT a
predictor of subsequent stock market performance," Morgan
Stanley's Adam Parker said in a Monday note to clients.
"Sometimes it means the market will roll over, sometimes it means
it will expand. Clearly, predicting the overall market level
isn't that easy. If it were, a lot of us wouldn't have our
current jobs."

Notably, while narrow market breadth is unusual, it's not unheard
of.

"There have been numerous periods in stock market history when
bundles of stocks trend away from the bulk of the index,"
Jefferies' Sean Darby observed in a November 30 note to clients.
"The 'Titans' referred to a small group of large conglomerates
while the 'Nifty Fifty' were a collection of growth companies
that had a growing international footprint. The 'dot coms' relied
on 'capital light' business models."

According to the money managers at Goldman Sachs Asset
Management, investors should consider this as they tweak their
portfolios.

"Broadening leadership in the US equity market from an unusually
narrow group of winners in 2015 could favor stocks currently
trading below the market multiple and companies that can generate
revenue growth," they said.